WASHINGTON — The threat of death-defying bacteria, stubborn organisms that refuse to be conquered by antibiotic medicines, is growing more alarming.Infectious microbes that used to be able to resist only one drug, such as penicillin or methicillin, now resist multiple drugs. Some can survive virtually every weapon in doctors' medicine cabinets.

"This is very worrisome," said Stuart Levy, a microbiologist at Tufts University in Boston. "In many cases, there might be only one or no drugs to treat (an infection). We are not keeping up with the bacteria."

Two troubling recent developments:

· Some bacteria have acquired the ability to "eat" the very antibiotic medicines that are supposed to eat them.

"Almost all the drugs that we consider as our mainline defense against bacterial infections are at risk from bacteria that not only resist the drugs but eat them for breakfast," George Church, a geneticist at Harvard Medical School in Boston, wrote in the April 4 issue of the journal Science.

· A lethal new form of tuberculosis, known as XDR-TB, that is virtually impossible to cure has exploded in Africa, Asia and Russia. There are a small number of cases in the United States.

These XDR-TB bacteria possess "such extensive drug resistance as to be nearly untreatable with currently available drugs," Sarita Shah, a epidemiologist at the Albert Einstein College of Medicine in New York City, reported in the Emerging Infectious Diseases journal.

Church led a team of scientists who discovered "super-resistant" bacteria dining on a diet of antibiotics at 11 different soil sites. The researchers tested 18 well-known antibiotics, including vancomycin, which is often called doctors' last line of defense against drug-resistant germs. None of the antibiotics kept the bacteria from multiplying, despite concentrations as much as 50 times higher than normal use."There is no easy solution," said Michael Kimmerling, an epidemiologist at the University of Alabama in Birmingham. "We need to understand what's going on."

Meanwhile, scientists have been tracking the spread of antibiotic resistant TB bacteria since an alarming surge in the 1990s, particularly among HIV patients with weakened immune systems.

Most of these TB germs are multi-drug resistant, or MDR-TB, because they resist many of the most popular medicines. The World Health Organization, based in Geneva, Switzerland, estimated that about 490,000 MDR-TB cases, including 116,000 deaths, occur each year, more than half of them in China, India and Russia.

In the past few years, an extremely-drug resistant strain, called XDR-TB for short, has exploded. XDR-TB infected about 27,000 people and caused about 16,000 deaths in 2006, the last year for which data is available, WHO reported.

The United States is not immune to the problem, which is particularly prevalent among immigrants. In 2006, the Centers for Disease Control and Prevention recorded 116 cases of MDR-TB and four cases of XDR-TB in this country. Incomplete data show two XDR-TB cases here in 2007.

"XDR-TB presents a global threat and a challenge to TB control activities in the United States," the CDC said in a statement. "Renewed vigilance is needed."

"If we don't act now, we are going really to risk a disaster of an enormous proportions," said Mario Raviglione, the director of WHO's Stop TB Department.

"This is not a joke," Raviglione said. "TB itself is a very serious disease. It implies suffering. It implies six months of treatment, a chance of dying. When you deal with MDR-TB, the chance of dying goes to 30 percent. When you are dealing with XDR-TB, the chance of dying is more than 50 percent."

In an extreme case in one South African community, 52 out of 53 XDR-TB patients died in 2006, WHO reported.

Another microbe raising grave concern is known as MRSA — for methicillin-resistant Staphylococcus aureus. It used to be curable by penicillin, but no more.

The antibiotic resistance problem arises because bacteria are constantly mutating, creating tiny changes in their genes. Even if drugs kill most of the bacteria in an infected cell, some altered microbes may remain unharmed and start multiplying.

"Only one bacterium out of a population has to survive in order to replenish that environmental niche with a complete new population of resistant organisms," Levy said.

"We're making progress in understanding the genetic and biochemical basis of resistance, but we've made little progress in how we're going to control it."

For information about antibiotic resistance: www.fda.gov/Fdac/features/795_antibio.html